This works on any of the Canon DIGIC III/IV DSLRs except the 1D* series. This means it will work for the 40D, the 50D, the 450D, the 500D, and the 1000D. Thanks to all the 5DMkII users who reported success. As it turns out, you have to power-cycle the 5DMkII before the shutter count value updates itself. It will not work on the 1D*, 5D, 10D, 20D, 30D, 300D, 350D, and 400D. It's not that I have anything against the owners of those cameras, but simply that the Canon SDK does not support retrieving the shutter count for them.